Laminating films are generally made from polyethylene, polypropylene, polyesters, polyimides, polyetherimides, polysulphones and the like. In general these polymer films have surfaces that are chemically inert and non-porous. These surfaces have low surface energies that cause them to be non-receptive to bonding with adhesives, printing inks, coating and the like. Typically the surface of these materials are subjected to a surface treatment to improve their bonding characteristics. One method for achieving a bond between a layer of polymer film and another layer of film, such as an adhesive film, is to form a laminate by using a primer adhesive, which prepares the surface of one of the layers of polymer film for bonding to the other layer of film. This method, however, generally requires the use of an adhesive having volatile organic solvents, which react with the surface of the polymer film to render it more receptive to subsequent bonding. The organic solvents, however, may pose both safety and health risks. It is desirable, therefore, to have a method of laminating films that will eliminate the use of such organic solvents. It is also desirable to have an in-line method for treating the surface of a film shortly before lamination to insure a strong bond.
A number of other methods are known for treating the surfaces of film layers to increase the bonding characteristics. These methods include (1) corona discharge; (2) etching by means of acid or plasma; and (3) flame treatment. Generally the latter two means are reserved for molded parts, while corona treatment is used when bonding sheets or strips of film. The object of the corona treatment is to improve the wetability of surface of the film to improve the ability of the film to bond to adhesives. In bonding two polymer webs it is recognized that both webs should generally be corona treated prior to thermal laminating the layers. A discussion of the surface treatment of polymer films and corona treating in general is found in Chapter 14, "Surface Treatment" pp 241-249 of Web Processing and Converting Technology and Equipment, (D. Satas, ed. 1984), Van Nostrand Reinhold Company, New York, N. Y.
Once the adhesive has been bonded to a surface of the polymer film, the laminate is typically bonded to another article. One particular application of a use of a laminate is in manufacturing of preinsulated electrical connectors and in particular preinsulated crimpable connectors. This type of connector has an insulating sheath bonded to a metal ferrule member. Typical connectors of this type are disclosed in U.S. Pat. Nos. 3,320,354 and 3,611,262. Tools such as the devices disclosed in U.S. Pat. No. 3,328,872 have been designed to crimp the connectors of the above patents onto wires. Such connectors have long been used to splice insulated wires particularly wires such as telephone cables.
In forming the insulating sheaths for such connectors, it is necessary to adhere the metal to a polymer film, such as a polyester film. In assembling such connectors the film layer is first laminated to a film of adhesive and then to a strip of metal that has been stamped into a series of connector blanks. The unit comprising the sheath, adhesive and metal is then formed into a U-shaped connector. The insulating sheath must have sufficient flexibility and bond strength to withstand the force exerted in forming the connector without delamination occurring between the adhesive film and the metal or the adhesive film and the outer film layer. In addition, the formed connector should be resistant to environmental changes, such as fluctuations in temperature and humidity, without delamination thereby assuring a long shelf life.
It is desirable, therefore, to have a means for assuring the bond between the outer sheath film and the adhesive film and the bond between the adhesive film and the metal that are capable of withstanding such forming forces as well as subsequent crimping forces, which may be in excess of 500 kilogram/square centimeter (7000 pounds/square inch).
In addition it is desirable to have a method for assuring a bond that does not require the use of volatile organic solvents.
The present invention is directed to a method for laminating film layers without the need of adhesive primer materials and to a method for bonding that assures a secure bond even when the materials are subjected to high tensile forces.